The “printed electronics technology,” which is known as a technology for manufacturing electronic devices by taking advantage of printing techniques (screen printing, flexographic printing, gravure printing and the like), has been attracting attention as a next-generation printing technology. This technology has been actively studied in recent years as it has a potential to meet those demands for lighter, smaller, more flexible and/or larger-area electronic devices as well as those demands such as cost reduction, mass production, energy conservation, chemical-usage reduction and the like. Indeed, the printed electronics technology has already yielded a wide variety of products in practical use such as membrane switches, wirings in RF-ID, displays, flexible solar cells, sensors and electronic papers, and will certainly create even larger markets and economic effects in the future.
Meanwhile, when the printed electronics technology is used to print an electronic circuit on a substrate with an electroconductive ink, a conventional electroconductive ink needs to be cured or dried by applying heat at about 50 to 120° C. at lowest. This means that a heating process is a mandatory requirement for conventional electroconductive inks.
Electroconductive inks which require heating, however, can not be applied to heat-sensitive substrates such as PET films. In view of the above, a challenge in the printed electronics technology is to develop an electroconductive ink which does not require heating so that circuits can be formed even on heat-sensitive substrates.
Accordingly, an electroconductive ink has been developed which can be cured only with ultraviolet light without applying heating after applied on a substrate to form an electronic circuit. An electroconductive ink curable only by the action of ultraviolet light can omit a heating process, and also provide a large number of advantages such as high productivity, low pollution, good work environment (VOC-free) and good printing quality (high hardness, weather resistance). As an example of such an ultraviolet-curable electroconductive ink, Patent Document 1 describes a composition including a polymerizable compound, a photosensitizer, an electrically conductive substance and saturated copolymerized polyester soluble in a predetermined phosphorus compound and the polymerizable compound, in which the polymerizable compound and the phosphorus compound are blended in a predetermined proportion.
As another example, Patent Document 2 discloses a composition as a UV-curable electroconductive ink suitable for flexographic printing, rotogravure printing and the like, the composition including one or more oligomers such as urethane acrylate, one or more acrylate carriers such as diacrylate and triacrylate, one or more reactive monomers such as vinyl ether, one or more electroconductive fillers such as flake-like silver powder and one or more photoinitiators.
Further, Patent Document 3 discloses an active energy ray-curable electroconductive ink composition as an electroconductive ink suitable for flexographic printing and screen printing, the composition including electrically conductive powder, an active energy ray-curable resin and a diluent as essential components, in which the active energy ray-curable resin consists of polyfunctional urethane acrylate.
Many of the conventional electroconductive inks, however, tend to cause transfer errors and blurrings when the content of an electroconductive filler is increased in an attempt to achieve a good electric conductivity. This may result in problems of printability, printing precision and the like such as no conductivity after post transfer-printing curing.
Moreover, an increased content of an electroconductive filler may prevent ultraviolet light from reaching the entire electroconductive ink, resulting in curability problems such as failed curing of a printed electroconductive ink at a deep portion.
In addition, a presence of ozone at the surface of a substrate, which may be generated from oxygen by the action of light having a wavelength of 220 nm or less, may inhibit curing when curing is performed with ultraviolet light, resulting in a problem of insufficient curing.
Accordingly, the present applicant previously disclosed a photocurable electroconductive ink composition in Patent Document 4 which overcomes the above problems, the compound including (A) an oligomer of urethane acrylate, (B) three types of acrylates consisting of any one of tetrafunctional acrylate or trifunctional acrylate, bifunctional acrylate and monofunctional acrylate, (C) an electroconductive filler, (D) two or more photopolymerization initiators selected from 1-hydroxycyclohexyl phenyl ketone, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, 2-methyl-1-(4-methylthiophenyl)-2-morpholinopropane-1-one, benzophenone and 2,4-diethylthioxanthone and (E) a polymer dispersant, in which the content of the electroconductive filler (C) is 77 to 85 mass % relative to the total mass of the photocurable electroconductive ink composition, and 80 mass % or more of the electroconductive filler (C) is silver powder in a scale-like, foil-like or flake-like form having a particle diameter at 50% particle size distribution of more than 5 μm.